Interlocking Reverse Hip Revision Prosthesis and Method

ABSTRACT

A reverse hip prosthesis comprising an acetabular cup for implanting in a previously implanted acetabular cup. The acetabular cup is secured to the previously implanted acetabular cup using an o-ring. The acetabular cup has a stem extending from the center of a concave portion thereof and an acetabular ball is attached to the stem, preferably by means of a Morse taper. A femoral implant is provided with a femoral cup attached to the proximal end thereof. The femoral cup has a stem which can be constructed in a modular fashion with several stem lengths in order to accommodate the various sizing requirements of different patients. After implantation of the acetabular cup and ball and the femoral cup, the members are assembled together so that the femoral cup can articulate on the acetabular ball. As the femoral cup is articulated, the edge of the femoral cup moves into a space located between the acetabular cup and the acetabular ball and thereby becomes constrained, thus reducing the likelihood of dislocation during extreme ranges of motion.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/024,381 filed Feb. 10, 2011 which is a continuation-in-partof U.S. Pat. No. 8,313,531 B2 which claimed the benefit of U.S.Provisional Patent Application Ser. No. 61/339,680 filed on Mar. 8, 2010and entitled “INTERLOCKING REVERSE HIP PROSTHESIS” and the entirety ofthe foregoing applications and patent are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to hip prostheses and morespecifically to an interlocking reverse hip prosthesis allowingincreased range of motion and stability during excessive range ofmotion.

2. Description of the Prior Art

It can be appreciated that several hip implants have been in use foryears. Typically, conventional hip implants comprise a femoral componenthaving an articular femoral ball attached to a stem. A femoral stem isinserted into the medullary canal of the femur after preparation andreaming using appropriate reamers by the operating surgeon. The stem canbe secured with bone cement or press fit. An acetabular component havingthe shape of a cup is inserted into an acetabular socket afterpreparation and appropriate reaming and secured with cancellous screwsthrough holes in the cup. It can also be secured with bone cement orpress fit or a combination thereof.

The acetabular cup is metallic and it is internally lined withhigh-density polyethylene or ceramic. Said lining is secured into theacetabular cup by a press-fit mechanism.

The main problem with conventional hip implants is the instability ofthe prosthesis at the extreme range of motion, thereby allowing thearticular femoral ball to dislodge and dislocate. Prior art teachesconstrained and preassembled ball and cup devices or devices wherein theball and cup members are implanted separately whereupon the ball elementis forced into a resilient opening in the cup and thereafter held inplace by the resilient material. Other constrained acetabular cups mayinclude a locking ring such as the one described by Albertorio et al.U.S. Pat. No. 6,527,808. In the case of cup elements having retainingrings, the ball member is forcefully inserted into the cup after the twoelements are implanted. This constitutes a weak link where forcesexerted on the prosthesis by ambulatory motion may exceed the forcesused to assemble the implant thereby causing the ball to be separatedfrom the cup.

While these devices may be suitable for the particular purpose whichthey address, they do not provide an interlocking mechanism as in thereverse hip implant design of the present invention. The very nature ofapplicant's design allows increased range of motion and increasedstability at extreme ranges of motion thereby reducing the risk ofdislocation.

In these respects, the interlocking reverse hip prosthesis according tothe present invention substantially departs from the conventionalconcepts and designs of the prior art because the articular femoral ballof the prior art is replaced with an articular femoral cup and theacetabular cup is provided with an acetabular ball. Thus an apparatus isprovided which is primarily developed for the purpose of reducing therisk of dislocation of hip implants at extreme ranges of motion.

Furthermore, since the articular surface of the femoral cup of theinvention is fully in contact 100% of the time with the surface of theacetabular ball, it is clear that this will improve tribology becausethe weight bearing distribution is improved on the articular surfaces,thus decreasing the wear of the surfaces in contact and reducing therisk of wear particles being released in the joint. The later, beingvery detrimental to the proper function of the joint.

SUMMARY OF THE INVENTION

The present invention provides a new interlocking reverse hip prosthesisconstruction wherein an acetabular ball is solidly and concentricallyattached to a central protrusion or stem of the acetabular cup via Morsetaper. A metallic acetabular cup is used in the preferred embodiment. Afemoral cup, also referred to herein as a hemispherical femoral cup oran articular femoral cup, is preferably solidly attached to a femoralimplant by means of a Morse taper. Other means of attachment known tothose in the art can be used. And whenever a Morse taper is referred toherein, it is intended to describe a preferred embodiment. The Morsetaper can be replaced by other suitable means of attachment as will beapparent to those having skill in the art.

The acetabular cup is implanted in an acetabular socket constructed bythe surgeon in the pelvic bone to which it is firmly secured by one ormore fasteners through one or more openings in the acetabular cup. Thefasteners can be cancellous screws or biocompatible resorbable studs ofvariable number. The femoral implant is then inserted and impacted intothe femoral medullary canal which has been prepared and hollowed by thesurgeon using appropriate reamers. During ambulation, the articularfemoral cup edge or lip will glide conformably and concentrically into aspace located between the acetabular ball and the acetabular cup. Aswill be apparent to those having skill in the art, the geometricalconfiguration of applicant's invention makes it very difficult for thefemoral cup to dislocate when the range of motion increases since itbecomes constrained in the locking space between the acetabular cup andthe acetabular ball.

As noted above, the articular surface of the femoral cup is fully incontact at all times with the articular surface of the acetabular ball.This improves the weight distribution, decreases the wear of thesurfaces in contact and reduces the risk of wear particles beingreleased in the joint.

In an optional embodiment of the invention, applicant has addressed therare possibility that soft tissue may get lodged in the implant in thespace between the acetabular cup and the acetabular ball. A protectivesheath can be used to avoid this possibility. As discussed in moredetail below, the sheath is disposed in the space between the acetabularcup and the acetabular ball and is allowed to glide freely therein.

There has thus been outlined the more important features of theinvention in order that the detailed description may be betterunderstood, and so that the present contribution to the art may bebetter appreciated. A novel feature of this invention is that thelocation of the articular surfaces of the hip joint, namely the ball andsocket, is reversed. This results in a new reverse hip implant which isnot anticipated, rendered obvious, suggested or even implied by anyprior hip prosthesis when considered alone or in any combination.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notcommitted in its application to the details of construction andarrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways aswill be apparent from the description herein to those having skill inthe art. Also, it is to be understood that the terminology employedherein are for the purpose of the description and should not be regardedas limiting. Moreover, the term “hemispherical” as it is used hereinmeans “having the shape of a portion of a sphere” and it is not limitedto one-half of a sphere but may be less than one-half of a sphere ormore than one-half of a sphere as will be apparent to those having skillin the art consistent with functionality.

To the accomplishment of the above and related objects, this inventionmay be embodied in the form illustrated in the accompanying drawings.However, the drawings are elicited only and changes may be made into anyspecific construction illustrated without departing from the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and advantages of the present inventionwill become fully appreciated as the same becomes better understood whenconsidered in conjunction with the accompanying drawings, in which likereference characters designate the same or similar elements throughoutthe several views, and wherein:

FIG. 1 is a perspective view of the interlocking reverse hip prosthesisof the invention.

FIG. 2 is a section view of the interlocking reverse hip prosthesis.

FIG. 3 is a section view of the interlocking reverse hip prosthesis inextension and external rotation.

FIG. 4 is a section view of the interlocking reverse hip prosthesis inflexion and internal rotation.

FIG. 5 is a perspective view of the prosthesis of the inventionillustrating an optional soft tissue protective sheath.

FIG. 5A is a perspective view of the protective sheath by itself.

FIG. 6 is a section view of the embodiment of FIG. 5.

FIG. 7 is a section view illustrating a femoral cup having a recessinstead of a stem for connection to a femoral implant.

FIG. 8 is a section view of the embodiment of FIG. 7 which has beenarticulated to an extreme position.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, in which the reference characters denotethe same or similar elements throughout the several views, FIGS. 1-4illustrate an interlocking reverse hip prosthesis, which comprises anacetabular cup (11) having a smooth concave surface and a convexnonarticular surface. The convex non-articular surface abuts a socket inthe pelvic bone when the acetabular cup is implanted in a patient andprovides a porous surface with multiple asperities and micro-voids toallow bone ingrowth. Furthermore, the acetabular cup (11) provides oneor more holes (12) at different locations for the purpose of using oneor more fasteners (14). The fasteners (14) can be screws or resorbablenonmetallic and biocompatible studs of different diameters and lengths.The studs, which can be called orthobiologic resorbable studs, willsecure the acetabular cup (11) during the initial phase of bone ingrowthand will resorb within one year, being replaced by newly generated boneand become part of the host pelvic bone. During that period, theacetabular cup (11) becomes solidly attached to the acetabular socket inthe pelvic bone (4) by bone ingrowth. The acetabular cup (11) has aconcave hemispherical surface in which a large acetabular cup stem (9)is disposed. The acetabular cup stem (9) has a male Morse taper forassembly to the acetabular ball (8) by means of the acetabular ballrecess (10) which has a female Morse taper. Referring to FIGS. 2-4 and6, the femoral cup (6) has a femoral cup stem (7) with a male Morsetaper while the femoral implant (1) has a cooperating femoral implantrecess (5) having a female Morse taper located in neck (3). FIGS. 7 and8 illustrate a femoral cup (20) with a neck (22) and a femoral cuprecess (21) having a female Morse taper. This cooperates with a femoralimplant stem having a male Morse taper (not shown). In a preferredembodiment, a modular system is used in a kit according to the inventionwherein the femoral cup stem (7) or neck (22) can come in differentlengths to accommodate the sizing needs of a patient. Therefore, in akit of the invention, two or more femoral cups are provided havingdifferent stem lengths. In a less preferred embodiment, the length ofthe neck (3) of femoral implant (1), or the femoral implant stem (notshown) can also have various lengths to accommodate patient sizing needsand in a kit comprising this embodiment two or more neck or stemcomponents having different lengths will be included. Other variationson the design to meet different sizing needs will be apparent to thosehaving skill in the art.

An important advantage of the present invention is that the greater theinterdigitation the more stability of the implant as opposed toconventional ball and socket hip implants, where increased range ofmotion is usually associated with increased risk of dislocation.

Referring to FIG. 2, the proximal femoral bone (2) is reamed in theusual fashion to accept a femoral implant (1) that can be cemented orpress fitted in the femoral medullary canal. The acetabular socket inthe pelvic bone (4) is reamed to the appropriate size to accept theacetabular cup (11), which is impacted for press fit at the correctangle of inclination and anteversion. Fasteners (14) in the form offixation screws or biocompatible resorbable studs are then inserted inplace to secure the acetabular cup (11). The acetabular ball (8) is thenaffixed onto the acetabular cup stem (9). FIG. 2 also illustrates centerline (C-C). In the position shown, the center line (C-C) passes throughthe center of the acetabular cup (11), the longitudinal center line ofthe acetabular cup stem (9), the center of acetabular ball (8), thelongitudinal center line of femoral cup stem (7) and the longitudinalcenter line of femoral implant recess (5). Obviously, when the femoralcup is articulated on the ball the center line associated with thefemoral components will not be colinear with the center line of theacetabular components. The line is simply illustrated in this way forconvenience.

Referring to FIGS. 3, 4 and 8, when the femoral cup (6) or (20)articulates on the acetabular ball (8), the edges of the femoral cup (6)or (20) move into and out of hemispherical space (16) and the articularsurface of the femoral cup (6) or (20) maintains the same area ofcontact with the acetabular ball (8) over the entire range of motion. Inother words, 100% of the articular contact area of the femoral cup (6)or (20) is maintained over the entire range of motion. FIG. 3illustrates the prosthesis of the invention in extension and externalrotation. FIG. 4 illustrates the prosthesis in flexion and internalrotation and FIG. 8 illustrates an extreme articulation position of thefemoral cup (20) on the acetabular ball (8).

In one embodiment, the articular surface of the femoral cup (6) or (20)contains a high molecular weight polyethylene lining of varyingthickness, but no less than 4 mm. In a different embodiment the liningcould be porcelain, ceramic or a metallic alloy.

An important feature of the present invention is the ability to placeacetabular ball (8) in a position that minimizes or eliminates tortionalforces on the acetabular cup and acetabular stem. This is illustrated inFIG. 3 wherein the acetabular ball (8) is affixed on acetabular cup stem(9) in a position wherein the equatorial plane (P-P) of the acetabularcup passes through the center (15) of the acetabular ball.

An optional embodiment of the invention illustrated in FIGS. 5-8 adds asoft tissue protective sheath (17) to the embodiments described above.The protective sheath, also illustrated by itself in perspective in FIG.5A, addresses the rare cases wherein soft tissue might get lodged in thespace (16) as a result of articulation of the prosthetic joint of theinvention.

Referring to FIGS. 6-8, the sheath (17) extends beyond the circularouter edge of the acetabular cup (11) and has a retaining ring (18). Thesheath (17) is installed by placing it into the acetabular cup (11)before the acetabular ball (8) is installed. The sheath (17) can have asolid surface as illustrated or it can be perforated with holes, slotsor the like having the same or different shapes and dimensions as may bedesired.

As can be seen from FIGS. 6-8, the sheath (17) is allowed to move freelywithin the space (16), restrained only by the acetabular ball (8) andacetabular cup stem (9). The sheath (17) also is moved by contact ofretaining ring (18) with the outer edge of femoral cup (6) or (20). Theretaining ring (18) will, for example, contact the outer edge of femoralcup (6) or (20) particularly in positions of extreme articulation of theprosthetic joint as illustrated in FIG. 8.

In another embodiment of the present invention, also illustrated inFIGS. 6-8, the acetabular cup (11) is designed for use in revisionsurgery of the hip. Revisions are surgical procedures where the existingimplant is removed. This most frequently requires removal of theacetabular cup and it is associated with a high level of morbidity. Theremoval of a previously implanted acetabular cup may be quite difficultsurgically, especially when the cup has metallic beads for boneingrowth. In these cases, the removal is also associated with iatrogenicbone loss leading to difficulty in inserting another conventionalacetabular cup.

There are instances where the acetabular cup was not implanted correctlyor where the lining of the existing implant becomes worn out and needsto be replaced. Recurrent dislocations of the hip implant are usuallysecondary to surgical misplacement of a conventional acetabular cup. Forexample, if during the initial procedure, the cup was placed either toovertical or retroverted (e.g., facing backward instead of forward).

To remedy the above cited complication, there are times where thesurgeon simply cements a conventional revision cup into the previouslyimplanted acetabular cup using conventional bone cement. However,problems arise if the initial position of the previously implantedacetabular cup is too vertical or retroverted, preventing a conventionalrevision cup from being glued in the previously implanted acetabularcup. As a result, removal of the previously implanted acetabular cupbecomes necessary—entailing significant risk and possible morbidity tothe patient as described above.

Another important advantage of the present invention is that the angleof inclination and retroversion are not critical since the interlockingmechanism of applicant's implant will compensate for the misalignment ofa previously implanted acetabular cup.

Most previously implanted acetabular cups have an insert in a concavesurface thereof. The insert provides a concave articulating surface forarticulation of the femoral ball. The insert, also referred to herein asa removable articulating surface, can be made from a plastic, such aspolyethylene, or any other suitable biocompatible material such asceramic. The insert is frequently held in place by means of acircumferential groove, also referred to herein as a secondcircumferential groove, located in proximity to equatorial plane nearthe circumferential edge of the concave surface of the previouslyimplanted acetabular cup. The circumferential groove may be engaged by aprotrusion or protrusions extending from the insert, especially aflexible plastic insert, into the groove. Alternatively, an o-ring issnapped into the groove to hold the insert in place. Thus, it isremovably affixed in the previously implanted acetabular cup by means ofthe second circumferential groove and a circumferential protrusionlocated in proximity to a circumferential edge on the outer or convexsurface of the insert, or an o-ring, wherein the circumferentialprotrusion or o-ring is held in the second circumferential groove.

The acetabular cup (11) of applicant's interlocking reverse hipprosthesis can optionally be provided with a thin circumferential groove(19), also referred to herein as the first circumferential groove,located in proximity of the equatorial plane of said cup as illustratedin FIGS. 6-8.

In revision surgery using the hip prosthesis of applicant's invention,the insert of the previously implanted acetabular cup is removed. Thecircumferential groove (19) of the interlocking reverse hip prosthesiswill host a retaining “o-ring” of the previously implanted acetabularcup being revised thereby providing solid fixation of applicant'srevision interlocking reverse hip prosthesis to the previously implantedacetabular cup.

The revision surgery is performed by removing the removable articulatingsurface and then placing the o-ring in either the first circumferentialgroove or the second circumferential groove. Then the acetabular cup ofthe interlocking reverse hip prosthesis is pushed into the previouslyimplanted acetabular cup until the o-ring snaps into place in both thefirst and second circumferential grooves, thereby solidly affixing theacetabular cup of the interlocking reverse hip prosthesis into thepreviously implanted acetabular cup.

The components of the reverse hip prosthesis of the invention are madefrom biocompatible materials commonly used in the art and suitablematerials will be apparent to those skilled in the art based upon thedisclosures herein. Metals or metallic alloys such as titanium or cobaltchrome are suitable. For some components, such as the acetabular ball,metals or ceramics can be used. High density polyethylene is alsosuitable for some components, for example the protective sheath or anoptional lining for the concave portion of the femoral cup. Otherbiocompatible materials or combinations thereof can be used for variouscomponents as will be apparent to those having skill in the art.

The dimensions of the various components of the reverse hip prosthesisof the invention can be readily determined by those skilled in the artbased upon the disclosures herein. For the acetabular cup, which ishemispherical, an outer diameter from about 35 millimeters (mm) to about65 mm will be suitable for most applications. The spherical acetabularball should have a diameter from about 28 mm to about 45 mm. Thediameter of the acetabular ball should be from about 7 mm to about 12 mmsmaller than the inner diameter of the acetabular cup, thus creating ahemispherical space or gap having a width from about 7 mm to about 12 mmto allow articulation of the femoral cup therein. Of course, theconcave, hemispherical, articular surface of the femoral cup will besized compatibly with the acetabular ball to allow for smootharticulation. Highly polished cobalt chrome is an excellent material forthe articular surface of the femoral cup but other materials such asbiocompatible metallic alloys can be used. The femoral cup also maycontain a lining fabricated from high-density polyethylene, ceramic orbiocompatible metallic alloys.

It is therefore the object of the present invention to provide a new andimproved interlocking and restrained reverse hip prosthesis system,where two conventional articular surfaces of the hip joint are reversedand interlocked. The system described in the present invention, whichhas all of the advantages of the prior art designs, none of thedisadvantages, and numerous improvements over the prior art,particularly in respect of remarkably increased range of motion andreduced risk of wear particles being released into the joint.

With respect to the above description then, it is to be realized thatthe optimum dimensional relationships for the parts of the invention, toinclude variations in size, materials, shape, form, function and mannerof operation, assembly and use, are deemed readily apparent to thoseskilled in the art, and all equivalent relationships to thoseillustrated in the drawings and described in the specification areintended to be encompassed by the present invention. Therefore, theforegoing is considered as illustrative only of the principles of theinvention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be resorted to, falling within the scope of the invention.

What is claimed is:
 1. A hip prosthesis having increased stability atextreme ranges of motion comprising a unitary acetabular cup having anon-articulating surface for attachment to an acetabular socket in apelvic bone and a concave surface located opposite to thenon-articulating surface, the non-articulating surface having acircumferential groove located in proximity to a circumferential edge ofthe acetabular cup and the concave surface having an acetabular cup stemfirmly affixed therein and projecting outwardly therefrom, an acetabularball firmly affixed to the acetabular cup stem, the acetabular ballhaving a surface, the concave surface of the acetabular cup and thesurface of the acetabular ball are spaced from one another, therebydefining a gap therebetween, a femoral implant for implantation in amedullary canal of a proximal end of a femur, and a femoral cup firmlyaffixed to a proximal end of the femoral implant, the femoral cup beingsized for articulation in the gap, such that the femoral cup has aconcave surface sized for articulation on the surface of the acetabularball and a convex surface opposite the concave surface of the femoralcup sized for articulation on the concave surface of the acetabular cup,the gap being sized and configured to permit said articulation whileconstraining the femoral cup within the gap throughout an entire rangeof said articulation of the femoral cup as it articulates within thegap, thereby reducing the risk of dislocation.
 2. The prosthesis ofclaim 1 further comprising an o-ring sized to fit in the circumferentialgroove.
 3. The prosthesis of claim 1 wherein the concave surface of thefemoral cup is hemispherical and the concave surface of the acetabularcup is hemispherical.
 4. The prosthesis of claim 1 wherein the concavesurface of the acetabular cup has a center and the acetabular cup stemis affixed to and in the center.
 5. The prosthesis of claim 1 whereinthe acetabular ball has an acetabular ball recess sized to receive theacetabular cup stem.
 6. The prosthesis of claim 1 wherein the femoralcup has a femoral cup stem projecting outwardly therefrom in a directionopposite the concave surface thereof and the femoral implant has at itsproximal end a recess sized to receive the femoral cup stem.
 7. Theprosthesis of claim 1 wherein the femoral implant has a femoral implantstem extending proximally therefrom and the femoral cup has a recesssized to receive the femoral implant stem.
 8. The prosthesis of claim 5wherein the acetabular ball has a center, the acetabular cup stem has alongitudinal center line and the acetabular ball recess has alongitudinal center line, both longitudinal center lines being colinearand passing through the center of the acetabular ball.
 9. The prosthesisof claim 6 wherein the femoral cup has a concave hemispherical portionhaving a center line, the femoral cup stem has a longitudinal centerline and the femoral implant recess has a longitudinal center linewherein all of the center lines are colinear.
 10. The prosthesis ofclaim 7 wherein the femoral cup has a concave hemispherical portionhaving a center line, the femoral implant stem has a longitudinal centerline and the femoral cup recess has a longitudinal center line whereinall of the center lines are colinear.
 11. The prosthesis of claim 1further comprising a hemispherical protective shield moveably disposedin the gap, the protective shield having a retaining ring for engagementwith a circumferential outer edge of the femoral cup wherebyarticulation of the femoral cup on the acetabular ball causes movementof the protective shield.
 12. The prosthesis of claim 1 wherein theconcave surface of the femoral cup is fully in contact with the surfaceof the acetabular ball during articulation of said concave surface onthe acetabular ball.
 13. A revision surgery method of implanting in apatient a hip prosthesis having increased stability at extreme ranges ofmotion comprising a unitary first acetabular cup having anon-articulating surface for attachment to an acetabular socket in apelvic bone and a concave surface located opposite to thenon-articulating surface, the non-articulating surface having a firstcircumferential groove located in proximity to a circumferential edge ofthe acetabular cup and the concave surface having an acetabular cup stemfirmly affixed therein and projecting outwardly therefrom, an o-ringsized to fit in the first circumferential groove, an acetabular ballfirmly affixed to the acetabular cup stem, the acetabular ball having asurface, the concave surface of the first acetabular cup and the surfaceof the acetabular ball are spaced from one another, thereby defining agap therebetween, a femoral implant for implantation in a medullarycanal of a proximal end of a femur, and a femoral cup firmly affixed toa proximal end of the femoral implant, the femoral cup being sized forarticulation in the gap, such that the femoral cup has a concave surfacesized for articulation on the surface of the acetabular ball and aconvex surface opposite the concave surface of the femoral cup sized forarticulation on the concave surface of the acetabular cup, the gap beingsized and configured to permit said articulation while constraining thefemoral cup within the gap throughout an entire range of saidarticulation of the femoral cup as it articulates within the gap,thereby reducing the risk of dislocation, the patient having an implantin need of revision, the implant comprising a hip joint prosthesis withan implanted second acetabular cup and a femoral stem having anarticulating femoral ball affixed to the proximal end thereof, theimplanted second acetabular cup having a concave articulating surfacewith a removable articulating surface therein, the removablearticulating surface being removably affixed in the second acetabularcup by means of a second circumferential groove in the concave surfaceof the acetabular cup, the method comprising removing the femoral stemand articulating femoral ball, removing the removable articulatingsurface, placing the o-ring in either the first circumferential grooveor the second circumferential groove, firmly affixing to the concavearticular surface of the implanted second acetabular cup to the firstacetabular cup by snapping the O-ring in place so that it is affixed inboth the first circumferential groove and the second circumferentialgroove, preparing a medullary canal at the proximal end of the femur andaffixing the femoral implant therein, affixing the acetabular ball tothe acetabular cup stem, affixing the femoral cup to the proximal end ofthe femoral implant and aligning the concave surface of the femoral cupwith the acetabular ball so that the concave surface of the femoral cupwill articulate on the acetabular ball.
 14. The method of claim 13further comprising disposing a protective shield in the acetabular cupprior to affixing the acetabular ball to the acetabular cup stem.
 15. Akit comprising the prosthesis of claim 6 and further comprising one ormore additional femoral cups, wherein the femoral cup stems havedifferent lengths.
 16. A kit comprising the prosthesis of claim 7wherein the femoral cup recess is disposed in a neck projectingoutwardly from the femoral cup in a direction opposite the concavesurface thereof, further comprising one or more additional femoral cups,wherein the femoral cup necks have different lengths.